Portable sampling spectrum analyzer

Abstract

A portable spectrum analyzer using frequency synthesis is used in the field to perform voltage amplitude measurements of specified channel segments in a multichannel configured coaxial cable environment. The instrument is a microprocessor (28) based system, and includes an RF input (1) and keyboard (36) and function switch inputs and provides both aural and visual outputs by means of a speaker and an LCD display (27). A three stage frequency converter (7) including PLLs (11, 12 and 13) controlled by the microprocessor (28) allows for the automatic monitoring of a plurality of carrier channels or the display of both video and audio levels of a single channel. Automatic adjustment of the input signal level is provided by attenuator pads (2 and 3) controlled by the microprocessor (28). Calibration is also automatically controlled by the periodic injection of a signal of predetermined level from a noise source (4) under the control of the microprocessor (28). The microprocessor is programmed to provide several modes of operation including a sweep mode, a zoom mode, and a memory mode. In the sweep mode, the instrument continually scans all carrier channels automatically, each channel being presented sequentially and simultaneously in bar graph form on the LCD display (27). In the zoom mode, a single channel is displayed with both video and audio levels side by side. In the memory mode, the operator can view data saved from measurements previously made at a selected location in the cable system.

What is claimed is: 
     
       1. A portable sampling spectrum analyzer for making field measurements in a multichannel configured coaxial cable communication system, comprising: a microprocessor;   memory means connected to said microprocessor for storing control code and data;   switch means connected to said microprocessor for providing operator input signals to said microprocessor;   signal selecting means connectable to a coaxial cable and controlled by said microprocessor for sampling in time a plurality of predetermined frequency channels of a coaxial cable communication system by selectively tuning a plurality of said channels at times determined by said microprocessor;   signal detection means connected to receive the output of said signal selecting means for storing sampled data in said memory means; and   visual display means connected to said microprocessor for displaying measured sampled data in at least one mode that displays data for all said predetermined frequency channels simultaneously.   
     
     
       2. The portable sampling spectrum analyzer recited in claim 1 wherein said microprocessor is programmed to provide a plurality of modes of operation including a sweep mode in which said visual display means displays video or audio signal levels of said plurality of frequency channels simultaneously. 
     
     
       3. The portable sampling spectrum analyzer recited in claim 2 where said microprocessor is further programmed to provide a zoom mode in which said visual display means displays sampled video and audio signal levels of a single frequency channel, the displayed video and audio signal levels being periodically updated. 
     
     
       4. The portable sampling spectrum analyzer recited in claim 3 wherein said microprocessor is programmed to provide a memory mode in which said visual display means displays previously measured data for a selected location in said coaxial cable. 
     
     
       5. The portable sampling spectrum analyzer recited in claim 3 further comprising means for selecting a single frequency channel to be displayed in said zoom mode and wherein said visual display means comprises: first display area for displaying said video and audio signal levels of a selected channel in bar graph form;   second display area for displaying channel number, frequency, carrier type, and signal strength for the sampled video signal; and   third display area for displaying type of carrier, the absolute signal strength of the audio signal, and the difference in the signal strengths of the video and audio signals.   
     
     
       6. The portable spectrum analyzer recited in claim 5 wherein said means for selecting comprises: means for displaying a cursor on said visual display means;   means for moving said cursor to a point on said visual display means corresponding to a signal frequency channel to be selected; and   means for providing an input signal to said microprocessor indicating the selection of a single frequency channel.   
     
     
       7. The sampling spectrum analyzer recited in claim 2 wherein said switch means includes cursor keys for controlling a cursor displayed by said visual display means, said microprocessor being responsive to the operation of said cursor keys in said sweep mode of operation for moving said cursor and providing a display of channel information for the channel selected by said cursor. 
     
     
       8. The portable sampling spectrum analyzer recited in claim 2 wherein said switch means includes a video/audio key, said microprocessor being responsive to the operation of said video/audio key in said sweep mode of operation for toggling between displays on said visual display means of video data and audio data for said plurality of frequency channels. 
     
     
       9. The portable sampling spectrum analyzer recited in claim 1 further comprising means for selecting a single frequency channel of said plurality of predetermined frequency channels and wherein said visual display means comprises: first display area for displaying the stored data for each sampled frequency channel in bar graph form to show relative signal strengths of said sampled frequency channels; and   second display area for displaying a channel number, frequency, type of carrier, and signal strength for the selected single frequency channel.   
     
     
       10. The portable sampling spectrum analyzer recited in claim 9 wherein said means for selecting a single frequency channel comprises: means for displaying a cursor on said visual display means;   means for moving said cursor to a bar graph corresponding to the channel to be selected; and   means for inputing a selection signal to said microprocessor.   
     
     
       11. The portable sampling spectrum analyzer recited in claim 1 further comprising attenuator means connected between the input of said signal selecting means and a test point on said coaxial cable and controlled by said microprocessor for automatically maintaining the input signal level within predetermined limits. 
     
     
       12. The portable sampling spectrum analyzer recited in claim 1 further comprising calibration means connected between the input of said signal selecting means and a test point on said coaxial cable and controlled by said microprocessor for substituting a noise signal of predetermined level for the input signal at said test point to automatically calibrate said spectrum analyzer. 
     
     
       13. A method of making field measurements in a multichannel configured coaxial cable communication system comprising the steps of: sampling in time a plurality of predetermined frequency channels of a coaxial communication system by selectively tuning a plurality of said frequency channels at determined times;   detecting signal strength on each sampled frequency channel and storing data corresponding to the detected signal strength for each sampled frequency channel in memory;   updating the data stored in memory for each sampled frequency channel by reading into memory the most recently detected signal strength for the sampled frequency channel; and   reading out the data stored in memory and displaying the stored data for each sampled frequency channel in bar graph form on a common display to show relative signal strengths of the sampled frequency channels.   
     
     
       14. The method of making field measurements as recited in claim 13 further comprising the steps of: selecting a single frequency channel of said plurality of predetermined frequency channels; and   displaying channel number, frequency, type of carrier, and signal strength for the selected single frequency channel.   
     
     
       15. The method of making field measurements as recited in claim 14 wherein the step of selecting is performed by the steps of: displaying a cursor on said common display;   moving said cursor to a bar graph corresponding to a single frequency channel to be selected; and   selecting the single frequency channel having a bar graph at the position of the cursor.   
     
     
       16. The method of making field measurements as recited in claim 13 wherein said steps of sampling is performed by sampling both a video signal and an audio signal and said step of displaying is performed in a sweep mode wherein data for all sampled frequency channels are displayed on a single display, said method further comprising the steps of: switching the mode of display to a zoom mode in which the sampled data for a single frequency channel of the coaxial cable communication system is displayed;   selecting a single frequency channel of said plurality of predetermined frequency channels for display of video and audio signal strengths; and   reading out sampled video and audio signal strength data from memory for the selected frequency channel and displaying the sampled video and audio signal levels of the selected frequency channel in bar graph form on the display.   
     
     
       17. The method of making field measurements as recited in claim 16 wherein the step of selecting is performed by the steps of: displaying a cursor on said common display;   moving the cursor to a bar graph corresponding to a single frequency channel to be selected; and   selecting the single frequency channel having a bar graph at the position of the cursor.   
     
     
       18. The method of making field measurements as recited in claim 16 further comprising the steps of: displaying channel number, frequency, carrier type, and signal strength for the sampled video signal; and   displaying the type of carrier, the absolute signal strength of the audio signal, and the difference in the signal strengths of the video and audio signals.   
     
     
       19. The method of making field measurements as recited in claim 13 wherein the steps of sampling, detecting signal strength and storing data corresponding to the detected signal strengths of said plurality of channels is performed at a plurality of locations in said coaxial cable communication system, said method further comprising the steps of: switching the mode of display to a memory mode and separately storing data corresponding to the detected signal strengths for said plurality of sampled frequency channels according to the location where the steps of sampling and detecting are performed; and   selecting the data stored in memory for one of said locations and displaying the data for said plurality of frequency channels in bar graph form on a common display, the data stored in memory mode in the memory remaining static.   
     
     
       20. The method of making field measurements as recited in claim 13 further comprising the step of automatically maintaining the input signal level for each of said predetermined frequency channels within predetermined limits. 
     
     
       21. The method for making field measurements as recited in claim 13 further comprising calibrating the measurements of detected signal strength for each of said predetermined frequency channels by performing the steps of: automatically substituting a noise source of known signal strength as the input signal to be detected; and   calibrating subsequent measurements to the measured signal strength of said noise source.   
     
     
       22. The method for making field measurements as recited in claim 13 further comprising the step of switching between a display of video data and a display of audio data for said plurality of predetermined frequency channels.